javascript - 如何在while循环中串联 promise

我必须对一系列的多个数据库进行查询。基本上是一系列必须完成的承诺。我正在一段时间内实现它。数据库是根据日期命名的。我还必须将请求延迟设置为400毫秒乘以每个承诺之间传递的循环数（400*count）。如果这有助于我使用cloudant作为我的数据库，类似于mongodb/couchdb，只是一个在线实例。
所以我实现了一个查询函数，它将获取开始日期和结束日期，并检索范围内的所有值。如果日期相同，则它将查询一个数据库，否则它将在日期范围内每天查询多个数据库。我对if语句中的else块主要有问题，如下所示。

db.predQuery = (start, end, locationCode) => {
        return new Promise((resolve, reject) => {
            let data = [];
            const startDate = moment(start, "MM/DD/YYYY");
            const endDate = moment(end, "MM/DD/YYYY");
            if (startDate.diff(endDate) === 0) {
                // THIS IF BLOCK WORKS GOOD
                let dbName = `prediction_${String(locationCode)}_`;
                dbName += startDate.format("YYYY-MM-DD");
                const db = this.cloudant.use(dbName);
                return db.find({selector: {_id: {'$gt': 0}}}).then((body) => {
                    data = body.docs;
                    return resolve(data);
                });
            } else {
                // THIS BLOCK IS WHERE THE PROBLEM IS

                // This is to start off the promise chain in the while loop
                let chainProm = Promise.resolve(1);
                let iterator = moment(startDate);
                let count = 0;

                // While loop for the series of promises
                while (iterator.isBefore(endDate) || iterator.isSame(endDate)) {
                    //dbName Format: prediction_0_2019-05-28
                    let dbName = `prediction_${String(locationCode)}_`;
                    dbName += iterator.format("YYYY-MM-DD");
                    const db = this.cloudant.use(dbName);
                    count += 1;

                    // Set off chain of promises
                    chainProm = chainProm.then(() => {
                        setTimeout(()=>{
                            db.find({selector: {_id: {'$gt': 0}}}).then((body) => {
                                // Keep adding on to the old array
                                data = data.concat(body.docs);
                            });
                        },count*400);
                    });

                    // Move to the next day
                    iterator.add(1,'days');
                }
                // Once all done resolve with the array of all the documents
                return resolve (data);
            }
        })
    };

基本上，输出应该是日期范围内所有文档的数组。现在，单一日期有效，但是当我做一系列日期时，要么请求没有通过，要么我达到了极限，要么它说承诺永远不会解决。我不认为这不是解决这个问题的最好办法，我对任何解决办法都持开放态度。任何帮助都非常感谢。

最佳答案

您的chainProm未连接到predQuery的外部调用-正在立即调用resolve。
您可能会发现使用async/await要容易得多，相反，循环内延迟逻辑将更容易理解：

const delay = ms => new Promise(resolve => setTimeout(resolve, ms));
db.predQuery = async (start, end, locationCode) => {
  const startDate = moment(start, "MM/DD/YYYY");
  const endDate = moment(end, "MM/DD/YYYY");
  if (startDate.diff(endDate) === 0) {
    // THIS IF BLOCK WORKS GOOD
    let dbName = `prediction_${String(locationCode)}_`;
    dbName += startDate.format("YYYY-MM-DD");
    const db = this.cloudant.use(dbName);
    const body = await db.find({selector: {_id: {'$gt': 0}}});
    return body.docs;
  }
  const iterator = moment(startDate);
  const data = [];
  const testShouldIterate = () => iterator.isBefore(endDate) || iterator.isSame(endDate);
  let shouldIterate = testShouldIterate();
  while (shouldIterate) {
    //dbName Format: prediction_0_2019-05-28
    let dbName = `prediction_${String(locationCode)}_`;
    dbName += iterator.format("YYYY-MM-DD");
    const db = this.cloudant.use(dbName);
    const body = await db.find({selector: {_id: {'$gt': 0}}});
    data.push(body.docs);
    // Move to the next day
    iterator.add(1,'days');
    shouldIterate = testShouldIterate();
    // Don't delay on the final iteration:
    if (!shouldIterate) {
      break;
    }
    await delay(400);
  }
  // Once all done resolve with the array of all the documents
  return data;
};

在这个实现中，任何错误都将发送给predQuery的调用方（错误将导致它返回的承诺被拒绝），因此在调用predQuery时，请确保在调用之后放置一个catch。